Development of Improved Suppression Measures Against Reignition Overvoltages Caused by Vacuum Circuit Breakers in Offshore Wind Farms

In offshore wind farms, reignition overvoltages are usually caused by switching-off operations of vacuum circuit breakers, which degrade the insulation of power equipment, especially transformers. This paper proposes a new quantitative evaluation method of overvoltage suppression measures comprehensively considering three main indicators, which include the amplitude, steepness and reignition number. Based on the quantitative evaluation results, an improved suppression measure against high-steepness reignition overvoltages is developed. Firstly, high-frequency transient models of main components of a typical wind farm are developed in PSCAD/EMTDC. Then the stochasticity of opening instants is considered, and three main indicators are extracted for comprehensively analyzing the transient characteristics of reignition overvoltages. Subsequently, a quantitative evaluation is performed to analyze the effectiveness of different suppression measures, which involves surge arresters, RC snubbers and smart chokes with different properties and installation positions. Results show that a new combination of a surge arrester and a smart choke is capable of providing sufficient protection against a high-amplitude, high-steepness and severe-reignitions overvoltage. This research can provide a new overvoltage suppression solution for the insulation coordination of medium-voltage systems in offshore wind farms.